runtime: badreflectcall runs on the G stack - convert to Go.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/136260043

src/pkg/runtime/proc.c

@@ -2617,12 +2617,6 @@ runtime·mcount(void)
 	return runtime·sched.mcount;
 }
 
-void
-runtime·badreflectcall(void) // called from assembly
-{
-	runtime·panicstring("runtime: arg size to reflect.call more than 1GB");
-}
-
 static struct {
 	uint32 lock;
 	int32 hz;

src/pkg/runtime/proc.go

@@ -101,6 +101,10 @@ func badmcall2(fn func(*g)) {
 	gothrow("runtime: mcall function returned")
 }
 
+func badreflectcall() {
+	panic("runtime: arg size to reflect.call more than 1GB")
+}
+
 func lockedOSThread() bool {
 	gp := getg()
 	return gp.lockedm != nil && gp.m.lockedg != nil